If you are a male barn swallow in the United States or the Mediterranean with dark red breast feathers, you’re apt to wow potential mates. But if you have long outer tail feathers in the United States, or short ones in the Mediterranean, the females may not be so impressed.

A new study led by the University of Colorado Boulder shows for the first time that differences in mate-choice decisions by female bird species among closely related populations can lead to the evolution of different physical traits. Such changes, the linchpin of evolution, often lead to speciation, or the formation of two or more different species from one, said Associate Professor Rebecca Safran, lead study author.

“The new twist here is we now have experimental evidence that the evolution of trait differences in closely related populations is being driven by female choice,” said Safran of CU Boulder’s Department of Ecology and Evolutionary Biology.

The study involved capturing barn swallows in Colorado and Israel with large nets and using non-toxic markers, clippers and feathers to alter the color of the breast feathers and either lengthen or shorten the outer tail feathers, called streamers. Individual males were each treated with one of five different combinations of breast color and/or streamer length. A control group of males were left unaltered in appearance.

Colorado barn swallows and those across America are characterized by darker breast plumage and shorter streamers, while the Israel swallows have lighter breast feathers and longer streamers. “We essentially gave the male barn swallows new outfits that mimicked the natural variation in color and streamer length in each population …

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